Surface runoff is a term used to describe the flow of water, from rain, snowmelt, or other sources, over the land surface, and is a major component of the water cycle. Runoff occurring on surfaces before reaching a channel is also called a nonpoint source. If a nonpoint source contains manmade contaminants, the runoff is called nonpoint source pollution. A land area which produces runoff draining to a common point is called a watershed. When runoff flows along the ground, it can pickup soil contaminants such as petroleum, pesticides (e.g., herbicides and insecticides), or fertilizers becoming discharge or nonpoint source pollution.
Urbanization increases surface runoff, by creating more impervious surfaces such as pavement and buildings, not allowing percolation of the water down through the soil to the aquifer. It is instead forced directly into streams or storm water runoff drains, where erosion and siltation can be major problems, even when flooding is not. Increased runoff reduces groundwater recharge, thus lowering the water table and making droughts worse, especially for farmers and others who depend on water wells.
When anthropogenic contaminants are dissolved or suspended in runoff, the human impact is expanded to create water pollution. This pollutant load can reach various receiving waters such as streams, rivers, lakes, estuaries and oceans with resultant water chemistry changes to these water systems and their related ecosystems. Further, there is considerable surface runoff in natural systems from animal wastes being entrained in runoff or from natural sediment loading in the absence of human alteration of the land. In underdeveloped countries the proportion of runoff attributable to natural factors has greater dominance, principally due to the lack of isolation of water supplies from potential runoff carrying animal waste.
Environmental issues associated with runoff include the impacts to surface water, groundwater, and soil through transport of water pollutants to these systems. Ultimately these consequences translate into human health risk, ecosystem disturbance, and aesthetic impact to water resources. Some of the contaminants creating the greatest impact to surface waters arising from runoff are petroleum substances, herbicides, and fertilizers. Quantitative uptake by surface runoff of pesticides and other contaminants has been studied since the 1960s, and early on contact of pesticides with water was known to enhance phytotoxicity, In the case of surface waters, the impacts translate to water pollution, since the streams and rivers have received runoff carrying various chemicals or sediments. When surface waters are used as potable water supplies, they can be compromised regarding health risks and drinking water aesthetics (e.g., odor, color, and turbidity effects). Contaminated surface waters risk altering the metabolic processes of the aquatic species they host; these alterations can lead to death, such as fish kills, or alter the balance of populations present. Other specific impacts are on animal mating, spawning, egg and larvae viability, juvenile survival, and plant productivity.
Storm water runoff from building roofs, parking lots, roadways, etc., picks up contaminants harmful to the environment if allowed to pass, untreated, into rivers, streams, aquifers and the like. The EPA has data suggesting polluted storm water runoff is a leading cause of impairment to the nearly 40% of surveyed U.S. water bodies which do not meet water quality standards. Over land or via storm sewer systems, polluted runoff is discharged, often untreated, directly into local water bodies. The pollutants may include solids including sand, gravel, grass, leaves, and the like. It is also known storm water runoff can pickup various nutrients, including phosphorous, potassium, and nitrates from lawn and agricultural fertilizers and heavy metals, including cadmium, zinc, copper, lead, nickel, chromium from metal building roofs, gutters, downspouts, and the like. Storm water runoff from roadways and parking lots include polycyclic, aromatic hydrocarbons from oils and motor fuels. Of course, significant concentrations of solids and dissolved pollutants should not be discharged to ground water or open watercourses. Also, the introduction of this water into the city sewer system is not desired, since a corresponding capacity must be reserved in this sewer treatment facility for this purpose.
In the case of groundwater, the main issue is contamination of drinking water, if the aquifer is abstracted for human use. Regarding soil contamination, runoff waters can have two important pathways of concern. Firstly, runoff water can extract soil contaminants and carry them in the form of water pollution to even more sensitive aquatic habitats. Secondly, runoff can deposit contaminants on relatively pristine soils, creating health or ecological consequences.
Mitigation of adverse impacts of runoff can take several forms: land use development controls aimed at minimizing impervious surfaces in urban areas; erosion controls for farms and construction sites; flood control programs; and chemical use and handling controls in agriculture, landscape maintenance, industrial use, etc.
Chemical use and handling has become a focal point mainly since passage of NEPA (National Environmental Policy Act) in the U.S. States and cities have become more vigilant in controlling the containment and storage of toxic chemicals, thus preventing releases and leakage. Methods commonly applied are: requirements for double containment of underground storage tanks; registration of hazardous materials usage; reduction in numbers of allowed pesticides; and more stringent regulation of fertilizers and herbicides in landscape maintenance. In many industrial cases, pretreatment of wastes is required to minimize escape of pollutants into sanitary or storm water sewers.
The U.S. Clean Water Act (CWA) requires local governments in urbanized areas (as defined by the Census Bureau) to obtain storm water discharge permits for their drainage systems. Essentially this means the locality must operate a storm water management program for all surface runoff entering the municipal separate storm sewer system. EPA and state regulations and related publications outline six basic components each local program must contain: public education (informing individuals, households, businesses about ways to avoid storm water pollution); public involvement (support public participation in implementation of local programs); illicit discharge detection and elimination (removing sanitary sewer or other non-storm water connections); construction site runoff controls (e.g., erosion and sediment controls); post-construction (i.e., permanent storm water management controls; and pollution prevention and “good housekeeping” measures (e.g., system maintenance). Other property owners which operate storm drain systems similar to municipalities, such as state highway systems, universities, military bases and prisons, are also subject to the permit requirements.
Surface runoff is not the only contaminate. Industrial process waters and contained water are polluted liquids providing potential hazards to the environment. Natural made contaminates, such as animal waste, also provide potential hazards to water supplies.
For liquids, such as water, loaded with solid particles and dissolved toxic substances, it is desirable to have a structurally simple filter element, performing as a purification system, which effectively removes the solid particles and particulate toxic substances from the liquid without great expense.